The present invention relates to calibration methods and apparatus. It finds particular application in calibrating xenon gas concentrations used in conjunction with computed tomography scans or examinations and will be described with particular reference thereto. It is to be appreciated, however, that the invention may find application in the calibration of xenon and other gas concentrations in conjunction with gas imaging systems, other medical diagnostic scanners, or the like.
Heretofore, xenon enhanced computed tomography has been utilized for diagnostic examinations, particularly in the derivation local blood flow maps. Xenon gas is absorbed into the blood. Because xenon is a noble gas, it does not form stable compounds with other elements. In concentrations under 50%, it is generally considered medically safe when used as an inhalant, although it does have mild narcotic effects. In higher concentrations, the xenon can be toxic and in very high concentrations, it can lethal.
Because xenon is about 200 times more absorptive of x-radiation than air, it has been used in imaging techniques which developed images based on patterns of absorbed xenon. Typically, a mixture of 30% xenon and 30% oxygen was breathed by the patient undergoing CT examination. Oxygen and xenon concentration sensors automatically adjusted the flow of gas from separate oxygen and xenon sources in order to maintain the concentration of xenon in the breathed gases substantially constant. A base line scan of the patient was made before the patient started breathing xenon containing gas, conventionally with the patient breathing room air.
After the patient started breathing the xenon gas mixture, a series of scans of each body section was taken at selected time intervals. The series of scans recorded the absorption of the inhaled xenon from the blood into each body section of the body tissues. The rate of xenon build up or absorption in each section over the series of scans was determined. The concentration of xenon in the blood was determined by measuring the xenon concentration in the end tidal exhaled gases in the respiration cycle corresponding to each scan. The xenon-blood partition coefficient and the xenon blood flow were calculated for each section from the absorption rate and blood xenon concentration data collected over the series of scans.
One drawback of these prior art techniques resided in the calibration of xenon concentrations. First, if the measurement of the xenon concentration in the breathing gas were grossly miscalibrated, the patient could breath an injuriously high concentration of xenon. Second, any inaccuracy in the end tidal exhalation measurements caused corresponding errors in the calculated partition coefficient .lambda. and flow maps.
The present invention provides an unambiguous technique which utilizes x-ray absorption to calibrate the xenon gas mixture delivery system and to confirm that safe concentrations of xenon will be delivered to the patient.